KASAN: slab-out-of-bounds Read in bpf_skb_change_tail

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 4e76528b Merge 4.14.81 into android-4.14
git tree: android-4.14
console output: https://syzkaller.appspot.com/x/log.txt?x=1298df7b400000
kernel config: https://syzkaller.appspot.com/x/.config?x=9e4a95e0186919ba
dashboard link: https://syzkaller.appspot.com/bug?extid=f6c3917a094a4d64fde2
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14627ad5400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f6c391...@syzkaller.appspotmail.com

random: cc1: uninitialized urandom read (8 bytes read)
audit: type=1400 audit(1542601418.614:9): avc: denied { map } for
pid=1786 comm="syz-execprog" path="/root/syzkaller-shm812481192" dev="sda1"
ino=16483 scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
tcontext=unconfined_u:object_r:file_t:s0 tclass=file permissive=1
audit: type=1400 audit(1542601424.184:10): avc: denied { prog_load } for
pid=4156 comm="syz-executor2"
scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
tcontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tclass=bpf
permissive=1
==================================================================
BUG: KASAN: slab-out-of-bounds in ____bpf_skb_change_tail
net/core/filter.c:2370 [inline]
BUG: KASAN: slab-out-of-bounds in bpf_skb_change_tail+0xa2c/0xb90
net/core/filter.c:2367
Read of size 8 at addr ffff8801c7c57e10 by task syz-executor2/4162

CPU: 0 PID: 4162 Comm: syz-executor2 Not tainted 4.14.81+ #6
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0xb9/0x11b lib/dump_stack.c:53
print_address_description+0x60/0x22b mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report.cold.6+0x11b/0x2dd mm/kasan/report.c:409
____bpf_skb_change_tail net/core/filter.c:2370 [inline]
bpf_skb_change_tail+0xa2c/0xb90 net/core/filter.c:2367
___bpf_prog_run+0x248e/0x5c70 kernel/bpf/core.c:1012

Allocated by task 350:
save_stack mm/kasan/kasan.c:447 [inline]
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc.part.1+0x4f/0xd0 mm/kasan/kasan.c:551
slab_post_alloc_hook mm/slab.h:442 [inline]
slab_alloc_node mm/slub.c:2723 [inline]
slab_alloc mm/slub.c:2731 [inline]
kmem_cache_alloc+0xe4/0x2b0 mm/slub.c:2736
getname_flags+0xc4/0x540 fs/namei.c:138
getname fs/namei.c:209 [inline]
do_unlinkat+0xc9/0x650 fs/namei.c:4070
do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 350:
save_stack mm/kasan/kasan.c:447 [inline]
set_track mm/kasan/kasan.c:459 [inline]
kasan_slab_free+0xac/0x190 mm/kasan/kasan.c:524
slab_free_hook mm/slub.c:1389 [inline]
slab_free_freelist_hook mm/slub.c:1410 [inline]
slab_free mm/slub.c:2966 [inline]
kmem_cache_free+0x12d/0x350 mm/slub.c:2988
putname+0xcf/0x100 fs/namei.c:259
do_unlinkat+0x16c/0x650 fs/namei.c:4113
do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the object at ffff8801c7c56600
which belongs to the cache names_cache of size 4096
The buggy address is located 2064 bytes to the right of
4096-byte region [ffff8801c7c56600, ffff8801c7c57600)
The buggy address belongs to the page:
page:ffffea00071f1400 count:1 mapcount:0 mapping: (null) index:0x0
compound_mapcount: 0
flags: 0x4000000000008100(slab|head)
raw: 4000000000008100 0000000000000000 0000000000000000 0000000100070007
raw: dead000000000100 dead000000000200 ffff8801da97e000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801c7c57d00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801c7c57d80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801c7c57e00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^
ffff8801c7c57e80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801c7c57f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches